In the field of medical diagnostics, in many cases, one or more analytes have to be detected in test samples of a body fluid, such as blood, interstitial fluid, urine, saliva or other types of body fluids. Examples of analytes to be detected are glucose, triglycerides, lactate, cholesterol or other types of analytes typically present in these body fluids. According to the concentration and/or the presence of the analyte, an appropriate treatment may be chosen, if necessary.
Generally, devices and methods known to the skilled person make use of sensor elements (e.g., test strips) comprising one or more test chemistries, which, in the presence of the analyte to be detected, are capable of performing one or more detectable detection reactions, such as optically or electrochemically detectable detection reactions. With regard to these test chemistries and methods related thereto, reference may be made, e.g., to J. Hoenes et al. (The Technology Behind Glucose Meters: Test Strips, Diabetes Technology & Therapeutics, Volume 10, Supplement 1, 2008, S-10 to S-26, to US 2009/0246808 A1, and to Habermiiller et al. ((2000), Fresenius J Anal Chem 366:560). For electrochemical detection of glucose, a review is provided, e.g., in Heller & Feldman (2008), Chem. Rev. 108: 2482. Measurement setups making use of a working electrode and a counter electrode and, optionally, a reference electrode, were, e.g., disclosed in WO 2007/071562 A1.
A common example for an amperometric glucose biosensor comprises a glucose dehydrogenase enzyme with the bound co-factor PQQ or FAD and a mediator as electron acceptor and electron shuttle in combination with noble metal or graphite electrodes. The mediator is often chosen to have low oxidation potential, so that the resulting electrode polarization is low enough to not oxidize other redox active components existing in a blood test sample, like ascorbic acid or uric acid. The disadvantage of those mediators is that, because of the described properties, they can be easily reduced by reducing agents, like ascorbic acid, contained in the blood test sample. In blood test samples from hospitalized patients, for which typically disposable test strip based professional blood glucose systems are used, the ascorbic acid concentration can be relatively high and reach levels comparable to the usual blood glucose concentrations. As a consequence, a high positively biased reading can result.
In case of enzyme systems with only temporally bound co-factors, the co-factor itself can act as an electron shuttle and, in principle, no extra redox mediator is required. An example of this is a glucose dehydrogenase enzyme with NAD as co-factor. By the enzyme reaction the NAD is reduced into NADH. Then, the NADH diffuses to the electrode and is oxidized to the radical cation. Also here, the resulting current can be measured as a measure for the glucose concentration. The disadvantage is that the NADH electrode reaction requires a high oxidation potential, which also leads to oxidation of interfering substances in the blood test sample, like ascorbic acid or uric acid. This causes high positive biased readings as well.
To overcome the above described problems, a system having two working electrodes and one reference electrode is known from WO 2005/045416 A1. The first working electrode is covered with a reagent that contains the enzyme system and the mediator as the active ingredients. The second working electrode is covered with the same reagent except the enzyme system. Both electrodes reagent layers are wetted by the same blood test sample filling a capillary measuring chamber above the electrodes. In a first phase, a voltage is applied between the reference electrode as the counter electrode and the second working electrode, which is covered with the enzyme free reagent. Then the responding current is measured. Thereafter, the voltage is applied to the reference electrode as the counter electrode and the first working electrode, which has the reagent with the enzyme system. Then the current is measured as well. If a reducing agent or electroactive component is present in the blood test sample, this would reduce the mediator or be directly oxidized at the working electrode. In both cases an interfering background current on both working electrodes is the result. The reaction with the glucose, on the other hand, only occurs on the electrode covered with the enzyme containing reagent. To get the final result, the difference of the currents measured on both working electrodes is calculated. By that calculation, the interfering effects from electrodes are extinguishing each other and, therefore, only the response from the glucose reaction remains. The limitation of this method is that both working electrodes add individual noise to the calculated results. Usually the resulting error is a geometrical sum of the individual errors. Therefore, the reachable test precision is limited. Another disadvantage with this solution is that a test strip with more than two electrodes increases the complexity and cost of the production process. In a similar approach, EP 2 767 826 A1 proposed a continuous glucose sensor comprising two working electrodes and a reference electrode, wherein the first working electrode is covered with an active enzymatic portion of a sensor membrane, whereas the second working electrode is covered with an inactive-enzymatic or non-enzymatic portion of the sensor membrane.
It is therefore an objective of the present disclosure to provide means and methods to comply with the aforementioned needs, avoiding at least in part the disadvantages of the prior art.